On 09/10/2019 12:32, Dave Martin wrote:
> On Wed, Oct 09, 2019 at 09:26:05AM +0100, Cristian Marussi wrote:
>> Introduce a new common utility function get_current_context() which can be
>> used to grab a ucontext without the help of libc, and also to detect if
>> such ucontext has been successfully used by placing it on the stack as a
>> fake sigframe.
>
> Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx>
>
Thanks
Cristian
>>
>> Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>
>> ---
>> v7 --> v8
>> - added new in v8 (splitted out from v7 05/11)
>> ---
>> .../selftests/arm64/signal/test_signals.h | 6 +-
>> .../arm64/signal/test_signals_utils.c | 31 ++++++
>> .../arm64/signal/test_signals_utils.h | 98 +++++++++++++++++++
>> 3 files changed, 134 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/arm64/signal/test_signals.h b/tools/testing/selftests/arm64/signal/test_signals.h
>> index 901521188202..4fd3ba01e3b1 100644
>> --- a/tools/testing/selftests/arm64/signal/test_signals.h
>> +++ b/tools/testing/selftests/arm64/signal/test_signals.h
>> @@ -88,8 +88,12 @@ struct tdescr {
>> /* optional sa_flags for the installed handler */
>> int sa_flags;
>> ucontext_t saved_uc;
>> + /* used by get_current_ctx() */
>> + size_t live_sz;
>> + ucontext_t *live_uc;
>> + volatile sig_atomic_t live_uc_valid;
>> /* optional test private data */
>> - void *priv;
>> + void *priv;
>>
>> /* a custom setup: called alternatively to default_setup */
>> int (*setup)(struct tdescr *td);
>> diff --git a/tools/testing/selftests/arm64/signal/test_signals_utils.c b/tools/testing/selftests/arm64/signal/test_signals_utils.c
>> index e8bbe36c2660..222148568adf 100644
>> --- a/tools/testing/selftests/arm64/signal/test_signals_utils.c
>> +++ b/tools/testing/selftests/arm64/signal/test_signals_utils.c
>> @@ -11,14 +11,19 @@
>> #include <linux/auxvec.h>
>> #include <ucontext.h>
>>
>> +#include <asm/unistd.h>
>> +
>> #include <kselftest.h>
>>
>> #include "test_signals.h"
>> #include "test_signals_utils.h"
>> #include "testcases/testcases.h"
>>
>> +
>> extern struct tdescr *current;
>>
>> +static int sig_copyctx = SIGTRAP;
>> +
>> static char const *const feats_names[FMAX_END] = {
>> " SSBS ",
>> " PAN ",
>> @@ -156,6 +161,20 @@ static bool handle_signal_ok(struct tdescr *td,
>> return true;
>> }
>>
>> +static bool handle_signal_copyctx(struct tdescr *td,
>> + siginfo_t *si, void *uc)
>> +{
>> + /* Mangling PC to avoid loops on original BRK instr */
>> + ((ucontext_t *)uc)->uc_mcontext.pc += 4;
>> + memcpy(td->live_uc, uc, td->live_sz);
>> + ASSERT_GOOD_CONTEXT(td->live_uc);
>> + td->live_uc_valid = 1;
>> + fprintf(stderr,
>> + "GOOD CONTEXT grabbed from sig_copyctx handler\n");
>> +
>> + return true;
>> +}
>> +
>> static void default_handler(int signum, siginfo_t *si, void *uc)
>> {
>> if (current->sig_unsupp && signum == current->sig_unsupp &&
>> @@ -167,6 +186,9 @@ static void default_handler(int signum, siginfo_t *si, void *uc)
>> } else if (current->sig_ok && signum == current->sig_ok &&
>> handle_signal_ok(current, si, uc)) {
>> fprintf(stderr, "Handled SIG_OK\n");
>> + } else if (signum == sig_copyctx && current->live_uc &&
>> + handle_signal_copyctx(current, si, uc)) {
>> + fprintf(stderr, "Handled SIG_COPYCTX\n");
>> } else {
>> if (signum == SIGALRM && current->timeout) {
>> fprintf(stderr, "-- Timeout !\n");
>> @@ -221,6 +243,15 @@ static inline int default_trigger(struct tdescr *td)
>>
>> int test_init(struct tdescr *td)
>> {
>> + if (td->sig_trig == sig_copyctx) {
>> + fprintf(stdout,
>> + "Signal %d is RESERVED, cannot be used as a trigger. Aborting\n",
>> + sig_copyctx);
>> + return 0;
>> + }
>> + /* just in case */
>> + unblock_signal(sig_copyctx);
>> +
>> td->minsigstksz = getauxval(AT_MINSIGSTKSZ);
>> if (!td->minsigstksz)
>> td->minsigstksz = MINSIGSTKSZ;
>> diff --git a/tools/testing/selftests/arm64/signal/test_signals_utils.h b/tools/testing/selftests/arm64/signal/test_signals_utils.h
>> index 5e3a2b7aaa8b..fd67b1f23c41 100644
>> --- a/tools/testing/selftests/arm64/signal/test_signals_utils.h
>> +++ b/tools/testing/selftests/arm64/signal/test_signals_utils.h
>> @@ -4,6 +4,10 @@
>> #ifndef __TEST_SIGNALS_UTILS_H__
>> #define __TEST_SIGNALS_UTILS_H__
>>
>> +#include <assert.h>
>> +#include <stdio.h>
>> +#include <string.h>
>> +
>> #include "test_signals.h"
>>
>> int test_init(struct tdescr *td);
>> @@ -17,4 +21,98 @@ static inline bool feats_ok(struct tdescr *td)
>> return (td->feats_required & td->feats_supported) == td->feats_required;
>> }
>>
>> +/*
>> + * Obtaining a valid and full-blown ucontext_t from userspace is tricky:
>> + * libc getcontext does() not save all the regs and messes with some of
>> + * them (pstate value in particular is not reliable).
>> + *
>> + * Here we use a service signal to grab the ucontext_t from inside a
>> + * dedicated signal handler, since there, it is populated by Kernel
>> + * itself in setup_sigframe(). The grabbed context is then stored and
>> + * made available in td->live_uc.
>> + *
>> + * As service-signal is used a SIGTRAP induced by a 'brk' instruction,
>> + * because here we have to avoid syscalls to trigger the signal since
>> + * they would cause any SVE sigframe content (if any) to be removed.
>> + *
>> + * Anyway this function really serves a dual purpose:
>> + *
>> + * 1. grab a valid sigcontext into td->live_uc for result analysis: in
>> + * such case it returns 1.
>> + *
>> + * 2. detect if, somehow, a previously grabbed live_uc context has been
>> + * used actively with a sigreturn: in such a case the execution would have
>> + * magically resumed in the middle of this function itself (seen_already==1):
>> + * in such a case return 0, since in fact we have not just simply grabbed
>> + * the context.
>> + *
>> + * This latter case is useful to detect when a fake_sigreturn test-case has
>> + * unexpectedly survived without hitting a SEGV.
>> + *
>> + * Note that the case of runtime dynamically sized sigframes (like in SVE
>> + * context) is still NOT addressed: sigframe size is supposed to be fixed
>> + * at sizeof(ucontext_t).
>> + */
>> +static __always_inline bool get_current_context(struct tdescr *td,
>> + ucontext_t *dest_uc)
>> +{
>> + static volatile bool seen_already;
>> +
>> + assert(td && dest_uc);
>> + /* it's a genuine invocation..reinit */
>> + seen_already = 0;
>> + td->live_uc_valid = 0;
>> + td->live_sz = sizeof(*dest_uc);
>> + memset(dest_uc, 0x00, td->live_sz);
>> + td->live_uc = dest_uc;
>> + /*
>> + * Grab ucontext_t triggering a SIGTRAP.
>> + *
>> + * Note that:
>> + * - live_uc_valid is declared volatile sig_atomic_t in
>> + * struct tdescr since it will be changed inside the
>> + * sig_copyctx handler
>> + * - the additional 'memory' clobber is there to avoid possible
>> + * compiler's assumption on live_uc_valid and the content
>> + * pointed by dest_uc, which are all changed inside the signal
>> + * handler
>> + * - BRK causes a debug exception which is handled by the Kernel
>> + * and finally causes the SIGTRAP signal to be delivered to this
>> + * test thread. Since such delivery happens on the ret_to_user()
>> + * /do_notify_resume() debug exception return-path, we are sure
>> + * that the registered SIGTRAP handler has been run to completion
>> + * before the execution path is restored here: as a consequence
>> + * we can be sure that the volatile sig_atomic_t live_uc_valid
>> + * carries a meaningful result. Being in a single thread context
>> + * we'll also be sure that any access to memory modified by the
>> + * handler (namely ucontext_t) will be visible once returned.
>> + * - note that since we are using a breakpoint instruction here
>> + * to cause a SIGTRAP, the ucontext_t grabbed from the signal
>> + * handler would naturally contain a PC pointing exactly to this
>> + * BRK line, which means that, on return from the signal handler,
>> + * or if we place the ucontext_t on the stack to fake a sigreturn,
>> + * we'll end up in an infinite loop of BRK-SIGTRAP-handler.
>> + * For this reason we take care to artificially move forward the
>> + * PC to the next instruction while inside the signal handler.
>> + */
>> + asm volatile ("brk #666"
>> + : "+m" (*dest_uc)
>> + :
>> + : "memory");
>> +
>> + /*
>> + * If we get here with seen_already==1 it implies the td->live_uc
>> + * context has been used to get back here....this probably means
>> + * a test has failed to cause a SEGV...anyway live_uc does not
>> + * point to a just acquired copy of ucontext_t...so return 0
>> + */
>> + if (seen_already) {
>> + fprintf(stdout,
>> + "Unexpected successful sigreturn detected: live_uc is stale !\n");
>> + return 0;
>> + }
>> + seen_already = 1;
>> +
>> + return td->live_uc_valid;
>> +}
>> #endif
>> --
>> 2.17.1
>>
>>
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